Non-equilibrium plasma, produced by a uniform glow discharge, is utilized for the surface treatment of polymer films, fabrics, wool, metal, and paper to improve the physical and optical properties of the surface. Such properties include printability, wetability, durability, and adhesion of coatings.
The non-equilibrium plasma is generated within a thin gap between two electrodes. The gap is generally less than about two millimeters. A high voltage is applied to an active electrode. The active electrode is encased within a dielectric barrier that can be a ceramic or glass to ensure uniformity of the discharge. A grounded, counter electrode is positioned opposite to the active electrode and can be in the form of a rotating drum or a flat plate. A plasma medium which can be helium is injected into the region between the two electrodes to generate the non-equilibrium plasma. The substrate, which can be in sheet form, is passed between the active and counter electrodes to be treated by the non-equilibrium plasma. At high processing speeds, difficulties have arisen in treating the substrate due to a laminar flow barrier created by air entrainment. The entrained air flow mixes with the gas that is used as a plasma medium to alter the composition of the plasma, as well as its chemical kinetics and stability.
It is known to inject the plasma medium gas between electrodes and toward the substrate. For instance, in U.S. Pat. No. 6,361,748 B1 a barrier electrode arrangement is disclosed in which a process gas or plasma medium, that is also used for cooling purposes, is injected between two electrodes and towards the substrate surface to be treated. U.S. Pat. No. 6,429,595 discloses two air cooled electrodes in which the plasma medium gas is injected between the electrodes through a porous ceramic that acts as a diffuser. In both of these patents, at high processing speeds, air would tend to be drawn into the plasma medium to alter its composition.
As will be discussed, the present invention solves this problem by utilizing plasma medium in such a manner as to inhibit air ingress into the electrode assembly.